The most common symptom for which patients seek medical help is pain. Pain can be classified as either acute or chronic. Acute pain results from immediate tissue injury and is self-limited. Acute pain is a natural defense mechanism in response to immediate tissue injury and functions to induce withdrawal from the painful stimulus and to prevent further use of the injured body part to allow the damaged tissue to heal. Acute pain is amenable to traditional pain medications and therapeutics. These include non-steroidal anti-inflammatory drugs (NSAIDs) and opioids. In contrast to acute pain, chronic pain is present for an extended period of time (e.g., three or more months) that persists in the absence of the tissue damage that initiated the pain response. Chronic pain can lead to significant changes in a patient's quality of life and functional ability.
Chronic pain is a significant medical problem for which the efficacy of current treatments is dismal. In the United States it is estimated that 40 million people suffer from chronic recurrent headaches; 35 million people are afflicted with persistent back pain; 20 and 2.1 million people have severe pain associated with osteoarthritis and rheumatoid arthritis, respectively; and 5 million people suffer from cancer-related pain.